1. Field of the Invention
The present invention relates to an image forming apparatus employing an electrophotographic system, such as a copy machine, a printer, or a facsimile machine, for example.
2. Description of the Related Art
As one development system of an image forming apparatus employing an electrophotographic process, there is a contact development system in which development is performed in a state in which a development roller, which is a developer carrier, is rotated in contact with a photosensitive drum, which is an image carrier. In the contact development system, the surface of the photosensitive drum wears due to contact with the development roller and thus performance worsens, leading to a decrease in the quality of formed images. Consequently, technology has been proposed whereby wear of the photosensitive drum due to contact with the development roller is prolonged by performing development by causing the development roller to contact the photosensitive drum only during a time period in which an electrostatic latent image of the photosensitive drum is developed.
In Japanese Patent Laid-Open No. 2006-292868, a configuration is proposed in which, in an inline color image forming apparatus, driving and stopping of a development roller, and contact with and separation from the photosensitive drum, are performed in coordination with the timing at which development is performed at respective stations. In an inline system, image forming stations that form images of respective color components are disposed in series on an intermediate transfer belt, and toner images of the respective color components are formed in an image forming region in the order first image forming station (abbreviated below as st1)→st2→st3→st4 in the conveyance direction of the intermediate transfer belt. In Japanese Patent Laid-Open No. 2006-292868, driving and stopping of the development roller of the respective image forming stations, and contact with and separation from the photosensitive drum, are controlled according to this order. The inline system is also referred to as a tandem system.
Here, because the respective image forming stations are provided individually as exchangeable and comparatively inexpensive process cartridges, it is difficult to completely eliminate variation, that is, mechanical variation such as variation in the positional relationship with the main body of the image forming apparatus, variation in drive source control, and so forth. Variation arises in the mechanism for causing contact and separation of a photosensitive drum and a development roller, for example. Assume a mechanism is adopted in which the development roller is biased such that the development roller contacts with the photosensitive drum, and the development roller is caused by a cam mechanism to separate from the photosensitive drum against this biasing force. In this case, assuming that a cam is in the image forming apparatus main body, and a cam follower is in a process cartridge, there is a possibility of variation in the distance between the cam and the cam follower. This variation leads to an offset in the timing of contact and separation of the development roller and the photosensitive drum, the offset occurring between image forming stations or between process cartridges, and the timing offset can cause image defects. For example, when the contact timing of the development roller is later than the leading edge of an image forming region on the photosensitive drum, a leading edge portion of an image is omitted, or image defects occur due to contact shock of the development roller. Also, when the separation timing of the development roller is earlier than the trailing edge of the image forming region on the photosensitive drum, an image defect that the image trailing edge is omitted will occur. Note that the image forming region on the photosensitive drum is a region where a latent image (and eventually a visible image using toner) is formed on the surface of the photosensitive drum according to the size of the recording medium on which printing is performed.
In order to prevent these adverse effects arising due to variation in the timing of contact or separation of the development roller and the photosensitive drum, in Japanese Patent Laid-Open No. 2006-292868, control of driving and stoppage, and contact and separation, of the development roller is caused to have a margin preceding an image forming guarantee time, as shown in FIG. 24. The margin is, for example, a surplus time for absorbing variation in the time required from the start of movement until actual contact in order to cause the development roller to contact against the photosensitive drum. When the development roller has been moved from a position separated from the photosensitive drum to a position contacted against the photosensitive drum, if the margin time has passed after the start of movement, the development roller is guaranteed to be in a state contacted against the photosensitive drum regardless of variation in timing between image forming stations. Accordingly, the time subsequent to passage of the margin time after the start of movement of the development roller serves as the image forming guarantee time, in which image forming of a visible image by a developer using toner or the like is guaranteed. In the example in FIG. 24, the development roller is contacted against the photosensitive drum at a timing t241, and this contact is accelerated by the time of an offset 1 relative to the image forming guarantee time. Also, following image forming, in order to guarantee image forming, separation of the development roller contacted against the photosensitive drum is started after passage of the image forming guarantee time. In FIG. 24, a time corresponding to an offset 2 is required until actual separation. The occurrence of image forming defects is prevented by performing image forming while allowing for this sort of offset.
Therefore, in the example in FIG. 24, the development roller and the photosensitive drum are contacted for a time longer than the image forming guarantee time by offset 1+offset 2. That is, because an image forming guarantee time allowing for offset is ensured, when performing image forming, in many cases it can be assumed that the development roller and the photosensitive drum are in contact for a long period at least as long as the time necessary and sufficient for image forming. As a result, there is the problem that wear of the photosensitive drum advances due to contact that is not intrinsically necessary for image forming, so the life of the process cartridge is shortened.